Page 81 - Basic physical chemistry for the atmospheric sciences

Chemical kinetics h7

4 Stated more strictly, the activation energy is equal to the total energies that the
reactants must possess in order for the reaction to occur. Thus , the rotational and
vibrational energies of the molecules, as well as their translational energies, can
contribute to the activation energy.
5 Chemists use the term homogeneous and heterogeneous differently than physicists.
For example, in cloud physics, a homogeneous process is one involving just one
substance (in any phase), and a heterogeneous process involves more than one
substance.
6 Do not confuse adsorption with absorption. The former term refers to the binding of
molecules to a surface, and the latter to the inclu sion of molecules into the interior of
another substance.
7 Radiocarbon dating of organic materials is based on the following principles. Carbon-
1 2 [i.e. carbon with a mass number ( = number of protons + number of neutrons) of
1 2 ] is the stable isotope of carbon. Carbon- 1 4 is unstable (i.e. , radioactive) with a
half-life of 5,700 yr. Because carbon- 14 is produced in the upper atmosphere, the
ratio of carbon- 1 4 to carbon- 1 2 in the atmosphere is constant (and is believed to have
been so for at least 50,000 yr). Carbon- 14 is incorporated into atmospheric carbon
dioxide , which is in turn incorporated , through photosynthesis, into plants. When
animals eat plants, the carbon- 14 is then incorporated into their tissues. While a plant
or animal is alive, it has a constant intake of carbon compounds, and it maintains a
ratio of carbon- 14 to carbon- 1 2 that is identical to that of the air. When a plant or
animal dies , it no longer ingests carbon compounds. Therefore , the ratio of carbon- 14
to carbon- 1 2 decreases with time, due to the radioactive decay of carbon- 14. Hence ,
the period that elapsed since a plant or animal or organic material was alive can be
deduced by comparing the ratio of carbon- 14 to carbon- 1 2 in the material with the
corresponding ratio for air.